Not Applicable
Not Applicable
This invention relates generally to resistive elements for use in systems for measuring the level of liquid in a vessel and in particular for the measurement of fuel quantity, and is more particularly directed toward a resistive element that may be used in the construction of a submersible sensor designed for installation in a fuel tank.
It is well known that the fuel tank of an automobile is a hostile environment for a sensor system. When electrical fuel level sensing systems were first developed, the transmitter units, which were intended to operate at least partially submerged in the fuel, were designed as potentiometric sensors of a wire-wound or metal foil type. A float arrangement was used to detect the liquid level, and by coupling the float to a sliding contact on the potentiometric sensor, a measurement system in the vehicle could track changes in resistance that occurred with variations in liquid level.
Unfortunately, the early potentiometric transmitter units were not durable enough to withstand the hostile environment. Breakage of wire in the wire-wound resistive sensor types, and peeling of foil in the metal-foil variety of sensor, often led to early failure of the sensing system. Of course, the deleterious effect of automotive fuel on the wire or metal foil tended to accelerate system wear.
Transmitter units manufactured using wire-wound or metal foil techniques were eventually replaced by resistive film screen-printed on a durable substrate, such as a substrate of ceramic material. In earlier versions of these screen printed sensors, a single or dual wiper moved along a printed resistive track in response to fuel level changes conveyed to the wiper by an associated float. The printed resistive track is typically deposited on a substrate of ceramic material or porcelain coated steel for durability. Of course, the printed resistive region, even though formed from glass frit in combination with precious metals, is still subject to wear due to friction with the wiper arm.
In a variation on the early sensors of the prior art, an example 100 of which is shown in FIG. 1, the wiper contacts are designed to slide across a network of conductors 101 rather than the resistive film 102 itself Designing the system so that the wiper makes contact with high metal content conductive regions provides a lower resistance path in operation.
The conductive regions 101 are also designed for durability and long life in the presence of hostile solvents such as gasoline, but the materials for these conductive areas must generally be selected from among an expensive group of candidate materials. Suitable metals include palladium, platinum, Gold, and silver, which can be combined into alloys that perform adequately in the intended environment.
A laser may be used to adjust, or trim, the thick film resistor to the required resistance value, by making a series of cuts, at appropriate points along the resistor.
The various processing steps, typically used in the manufacture of the prior art resistor element of FIG. 1, are illustrated in FIG. 2. Conventionally, a number of resistor elements are typically fabricated on a single substrate. To facilitate their subsequent separation, the ceramic substrate is initially scribed 201, for example by a laser scribing process. The conductive tracks are deposited 202, using a conventional thick film screen printing process. The tracks are dried in an oven and then fired 203 in a furnace. Resistor material is then deposited, using a conventional thick film printing process 204, over and between the conductive tracks. The resistor material is then dried in an oven and subsequently fired 205 in a furnace. The resistor is then laser trimmed 206 to the required resistance value, by making a series of cuts into the resistor, at appropriate points along the resistor. The location and size of cut is determined by reference to measurements made of the resistor value, facilitated by a series of test pads 108 formed with the conductive tracks.
The previously scribed ceramic is then broken 207 into individual resistor elements by breaking along the previously scribed lines and finally the individual elements are tested, packed and shipped to customers (shown as a single step 208).
The conductive traces are arranged such that the wiper contact will only contact the conductive traces in one or more wiper contact areas (identified in dashed outline in FIG. 1 as reference numerals 104a, 104b) over the working sweep of the wiper. In practice, two concentric wiper contact areas 104a, 104b may be used to reduce contact noise and increase sensor reliability. The wiper cannot directly connect with the thick film resistor 102 as the contact resistance would be excessive and the wiper contact would wear away before achieving the required number of life cycles demanded by system specifications.
The conductive traces 101 generally consist of precious metal alloys such as Palladium-Silver and to a lesser degree Gold-Platinum or Gold Platinum-Palladium. These alloys are resistant to the thick film manufacturing process and to subsequent long-term exposure to various fuels. They are also formulated to be sufficiently hard to withstand the wear associated with hundreds of thousands of cycles of the wiper contact. The alloying elements, which are used to impart the hardness properties to either the gold or silver conductors, are selected from the Platinum Group of Metals (PGM) and in particular, Palladium. The addition of Palladium to Silver also mitigates against the tendency of silver to form ions in the presence of moisture, and physically migrate between conductive tracks, under the influence of an electrical potential. This phenomenon, known as metal migration, is minimized as the proportion of Palladium in the alloy is increased. The Platinum Group Metals are expensive elements and significantly contribute to the overall material cost of the resistor element.
In recent years, increased environmental legislation has resulted in efforts to significantly reduce the sulphur content in automotive fuels. The process of removing the sulphur from fuels is known to leave residual traces of highly reactive sulphur compounds behind. These sulphur compounds have been found to react with the silver in palladium/silver traces on conventional fuel sensor elements forming non-conducting silver sulphide deposits, which can lead to sensor failure.
Consequently, a need arises for a resistor element that is suitably rugged for fuel tank applications, that minimizes the exposure of silver alloy traces, and has a reduced requirement for Platinum Group Metals.
These needs and others are satisfied by the present invention, in which a variable resistive element is provided for use with at least one associated sliding electrical contact. The variable resistive element comprises a substrate, a first conductor pattern deposited on the substrate, at least one resistive region making electrical contact with the first conductive pattern. The first conductor defines a contact area for the associated sliding electrical contact and in which portion the conductor pattern is plated with a first plating. The first plating may be nickel or a nickel alloy. The substrate is preferably ceramic.
In accordance with one aspect of the invention, the resistive element is incorporated in a sensor element, which further comprises a wiper arm having at least one electrical contact for contacting the resistive element. Preferably, the electrical contact is nickel or palladium nickel. The sensor element is particularly suited for use as a fuel card sensor element in a fuel sensor. The fuel level sensor may further include a wiper arm having at least one sliding electrical contact movable along a contact area of the first conductor pattern of the resistive element, and a float arrangement coupled to the wiper.
In accordance with a further aspect of the invention, the variable resistive element further comprises a protective layer, which substantially covers the resistive element. The protective layer may be a polymeric material or a low temperature glass material. The protective layer may also cover sections of the conductor pattern.
In accordance with yet another aspect of the invention, the portion plated of the resistive element plated with the first plating is further plated with a second plating. The second plating may be gold or a gold alloy.
In accordance with a second embodiment of the invention, a method of manufacture of a resistive element is provided, the method comprising the steps of providing a substrate, the substrate having a pattern of conductive traces fixed thereon and at least one region of resistive material in contact with the pattern of conductive traces, and plating at least one section of the first pattern of conductive traces with a first plating. The first plating may be nickel or a nickel alloy. The plating process may be an electroless plating process.
In accordance with one aspect of this second embodiment, a further step of applying a protective layer to substantially cover the resistive material prior to plating step may be provided.
In accordance with a third embodiment of the invention, a variable resistive element is provided comprising a substrate, a first conductor pattern disposed on a surface of the substrate, at least one resistive region making electrical contact with the first conductive pattern, wherein at least one area of the first conductor pattern comprises a layer of nickel or nickel alloy. The substrate may be ceramic.
In one aspect of the third embodiment, the first conductor pattern of the variable resistive element may include a layer of silver compound material positioned between the layer of nickel or nickel alloy and the substrate. The layer of nickel or nickel alloy may be covered with a further metal layer. The further metal layer may be gold or a gold alloy.
In a further aspect of the invention, a sensor element is provided having the resistive element and including a wiper arm having at least one electrical contact for contacting the resistive element, wherein the contact portion of the electrical contact is nickel or palladium nickel.
In one further aspect of the invention, the variable resistive element further comprises a protective layer substantially covering the resistive element. The protective layer may be a plating resistant polymeric material or a plating resistant glass material. Preferably, the glass may be a low tmperature glass. The protective layer may also cover sections of the conductor pattern.